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Question: Cheap pure sine wave genertor 1khz?
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trobbins:
The RightMark plot shows raw harmonic performance, as I was also able to reproduce.

However, if then the signal source is adjusted to add nulling harmonic levels (as per link in earlier email, and repeated below) and the spectrum shows a particular harmonic descending to the noise floor then is the harmonic now suppressed to circa -130dB below fundamental, or is there a disconnect somewhere ?

https://dalmura.com.au/static/Loopback%20distortion%20trim.JPG
rhb:

--- Quote from: MasterT on January 14, 2020, 04:41:58 pm ---What you described is a pure oversampling, based on sqrt(S/N), when averaging 4 samples improves SNR by 2. It's only applicable with random Gaussian noise.  THD is different species, it does not go down with averaging, or increasing fft size. And adc noise is not random.

--- End quote ---

Yes, grasshopper.  The Gaussian random noise goes down.  The THD and other noise processes do not.  I am glad you have grasped my point.

This is very useful as it allows us to  to address the non-Gaussian random and correlated noise processes.  Of which there are many.  Each of which requires appropriate measures. And in many cases, a significant amount of work  simply to identify and very often years of effort to eliminate.  But eventually one gets to the point that precise and accurate measurements can be made despite the noise.  At least in theory.  Having spent many years fixing bugs and dealing with weird data, I suspect that much of the time the idea is correct, but the implementation is wrong.

It's rather like an onion.  There are many layers.   But before you can reach one, you have to remove all the outer ones.

Most of the EE community has a view of DSP which is 10-20 years out of date.  The oil industry was investing millions on  DSP  for seismic processing before digital data could even be collected.  They knew digital data was coming.  They were paying for it.  TI was created to build recording equipment for GSI, their parent company. 

My PhD supervisor at Austin had been head of research at GSI before he left for academia.  He was one of the 7 members of the Geophysical Analysis Group consortium working under Norbert Wiener with funding from the oil industry in the early 1950's.  Enders Robinson and Sven Treitel, whose papers were collected as the "Robinson & Treitel Reader"and eventually merged into a book, were the major apostles as my supervisor had very poor writing skills.

Sadly, even within Amoco, which employed Sven, things were often done incorrectly.  I ran into this a few months into my career in seismic.  I called Sven at the labs in Tulsa to ask him about it.   He told me I was correct. And added, "But the prophet is often not believed in his own country", or words to that effect.

In summary, yes, it can be done.  But you have to know how.

Havfe Fun!
Reg
RoGeorge:
It doesn't matter if the noise is Gaussian or not, but it must be uncorrelated with the measuring signal, and must have a zero average for the integrating time.
MasterT:

--- Quote from: trobbins on January 15, 2020, 02:03:25 am ---The RightMark plot shows raw harmonic performance, as I was also able to reproduce.

However, if then the signal source is adjusted to add nulling harmonic levels (as per link in earlier email, and repeated below) and the spectrum shows a particular harmonic descending to the noise floor then is the harmonic now suppressed to circa -130dB below fundamental, or is there a disconnect somewhere ?

https://dalmura.com.au/static/Loopback%20distortion%20trim.JPG

--- End quote ---
What the point in generation distorted signal, that eliminates distortion introduced by adc later on?  Makes you happy? This is not what this topic about - to generate clean sine wave.
Objective is synthesis pre-distorted digital data that would wipe out non-linearity of the DAC Obviously, you need right tool, THD meter that capable to see results. And something like TI's  ads127l01 , sadly I hate a package.
trobbins:
MasterT, the point is that the DAC of a soundcard can be set up to generate a much lower distortion signal than can be generated by just the raw DAC output signal of the soundcard when asked to generate a sinewave.

How do you then measure the DAC's performance?  Well that is the point of the loopback test, but as indicated earlier that then includes a distortion contribution from the soundcard ADC.  So the loopback spectrum plot shows that the DAC plus ADC performance can exhibit a measured distortion profile where the HD components are down at the noise floor level.  For a 15 year old soundcard, that noise floor level is circa -130dB below the fundamental, but it appears you don't have confidence in that measurement process (and have yet to explain that succinctly imho).

If a much better, and more expensive ADC was available for making measurements, then that should be able to gauge what the actual soundcard DAC harmonic distortion profile really is.  And yes maybe some further tweaking of the added harmonic distortion magnitudes/phases (in REW) could achieve a better null.  Unfortunately I don't have access to such equipment, so can't confirm the actual performance.  But it does appear to provide the cheapest bang for your buck solution imho.
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